Completely sacred we hear (6)
I believe the answer is:
wholly
'completely' is the definition.
(synonyms)
'sacred we hear' is the wordplay.
'sacred' becomes 'holy' (similar in meaning).
'we hear' indicates a 'sounds like' (homophone) clue.
'holy' is a homophone of 'WHOLLY'.
(Other definitions for wholly that I've seen before include "Absolutely!" , "Totally" , "in full?" , "Entirely, fully" , "in every respect" .)